I keep trying to eat meat but it just won’t cooperate!

The payments can become a legal liability for the processors. I believe there are federal laws that have penalties for anyone who facilitates transactions for certain prohibited goods or services. It’s the same reason cannabis shops have such a hard time getting payment and banking services.

The payment processors have very little incentive to take risks here. As others have noted, there isn’t much competition pressure.

EDIT: I went to find a source, and found the cannabis analogy isn’t right. Seems that Visa and MasterCard really are the primary censors of the porn industry. This archived FT article went in depth. https://archive.ph/zXKuD